1. Field of the Invention
This invention relates to the transfer of electrostatically formed toner images using an intermediate transfer member. In particular, it relates to creation of multi-color toner images with small particle toners.
2. Description of the Prior Art
The use of an intermediate transfer member is useful in electrophotography for a number of reasons, including simplified receiving sheet handling, single pass duplexing, saving wear on photoconductors and superposition of images to form multi-color images. Typically, a toner image is created on a photoconductive member electrophotographically, and is then transferred to an intermediate transfer member, such as a roller or web. For example, a negatively charged toner image is transferred from a photoconductor having an electrically grounded backing electrode, to an intermediate web or roller biased to a strong positive polarity. The toner image is then transferred from the intermediate member to a receiving sheet under the influence of a second electric field. The second electric field can be created, without changing the voltage on the intermediate member, by placing a roller or corona behind the receiving sheet, which is biased in a stronger, positive direction.
The most desirable use of intermediate transfer is for creating multi-color images. When an intermediate transfer member is used, two, three, four or more separate images of different color can be transferred in registration to the intermediate transfer member to create a multi-color image. The multi-color image can then be transferred in one step to the receiving sheet. This system has a number of advantages over the more conventional approach to making multi-color images in which the receiver sheet is secured to the periphery of a roller and rotated repeatedly into transfer relation with the photoconductor to receive the color images directly. The most important advantage is that the receiving sheet itself does not have to be attached to a roller. Attaching the receiving sheet to a roller has been a source of misregistration of images due to independently transferring each color image to the receiver, as well as complexity in apparatus. Other advantages, such as wear and tear on the photoconductive member and a straight and simple receiving sheet path are also important.
High resolution in electrophotographic color printing is desirable. In order to obtain higher resolution, fine toners are necessary. Toners less than 20 microns, and especially toners less than 10 microns in size, give substantially improved resolution in color imaging with high quality equipment. Unfortunately, fine toners are more difficult to transfer electrostatically than are traditional coarse toners. This is a problem using both single transfer and intermediate transfer members.
Rimai and Chowdry have shown that by avoiding air gaps between toner and receiver, the surface forces can be at least partially balanced, thereby permitting images made using small toner particles to be transferred using high efficiency. See Rimai and Chowdry, U.S. Pat. No. 4,737,433. See, also, Dessauer and Clark, Xerography and Related Processes, page 393, Focal Press (New York), N. S. Goel and P. R. Spencer, Polym. Sci. TechnoI. 9B, pp. 763-827 (1975).
When transferring toners having a volume weighted average diameter less than 12 microns, and using electrostatics at both transfers, a number of transfer artifacts occur. For example, a well known artifact called "hollow character" is a result of insufficient transfer in the middle of high density toned areas, e.g., in alphanumerics. Another artifact, "halo" is experienced when toner fails to transfer next to a dense portion of an image. Use of materials suggested in the prior art tends to produce these artifacts when using two electrostatic transfer steps. These problems cannot be eliminated merely by an increase of the transfer field, since that expedient is limited by electrical breakdown.
One attempt to solve this problem is disclosed in Rimai et al, U.S. Pat. No. 5,084,735. This patent discloses use of an intermediate transfer member with a compliant intermediate blanket overcoated with a thin skin which has a higher Young's modulus than the underlying blanket. The blanket gives compliance whereas the skin controls adhesion. The outer skin has Young's modulus greater than of 5.times.10.sup.7 Pa. A problem encountered when using a thin outer skin on the intermediate transfer member, is that it cracks and delaminates. Moreover the skin tends to reduce the compliance of the intermediate.